The invention broadly relates to a Raster Output Scanner (ROS) imaging system, and, more particularly, to a means and method for generating timing signals responsive to the detection of a scanning beam crossing a fiber optic detector.
In conventional ROS systems, an intensity modulated light beam generated by a gas or diode laser is repetitively scanned across the surface of a photosensitive image plane to form a latent image of a document or the like represented by input binary data. Each scan line comprises composite images of individual pixels representing on and off states of the laser. These pixels must be aligned from scan to scan in the vertical or fastscan direction; failure to do so results in the phenomenon known as scan line "jitter". It is known in the prior art that photodetectors can be positioned in the scan path a predetermined distance upstream from the recording surface where their output is used to generate a Start Of Scan (SOS) signal controlling the timing of the laser modulation wave form. Exemplary of the known detectors is a slit detector design in which the amplitude of the photodiode output signal is compared with a predetermined fixed reference voltage. When the scanned laser beam passes over the photodetector surface, the amplitude of the output signal reaches this reference threshold and an SOS pulse is generated. Also known is the so-called differential input or split detector which utilizes dual photodiode elements in very close proximity in an electronic comparator configuration that compensates for variations in scanning beam power. In operation, the sweep of the beam over the first detector establishes a dynamic reference level for the second detector that is proportional to the intensity of the scanned light beam. With this arrangement, the comparator is triggered when the swept beam is positioned at the midpoint between the detectors and the light levels in both detectors match exactly. An example of a split detector is disclosed in U.S. Pat. No. 4,386,272.
For many high speed, high resolution Raster Output Scanner (ROS) systems, a solid state laser diode or a HeNe laser is the preferred device for generating the recording beams. As is well known, the power output of these lasers varies in amplitude over time. The conventional slit detector, when used with a laser scanning system, is subject to jitter because the output current of the photodetector responds proportionately, in amplitude, to the Gaussian shape of the scanned beam as it sweeps across the face of the detector. Outputs produced by beams of different power levels will, necessarily, reach the fixed reference level at different relative times, resulting in SOS outputs at different times relative to passage of the center of a scanned Gaussian beam. Since the synchronization of the electronic system that controls the timing of the information bit stream defining the laser modulation wave forms for each line is keyed to the SOS pulse, this differential triggering effects a net translation of the exposure pattern of each scan line in the fast scan direction. As a result, the alignment of picture elements in the exposure raster from line to line is inexact.
The differential or split detector generates an SOS output when the Gaussian beam is centered between the two photodetector sites. Since the response depends only on the relative position of the beam and not on a specific amplitude level, the SOS output signal timing is independent of the beam power. In other words, the split detector generates an SOS signal which does not vary in time when the diode intensity changes. Both the slit detector and the split detector are typically configured in the same fashion; the photodetector elements and associated amplifiers and pulse shaping electronics are assembled in a remote housing which is positioned adjacent to the imaging surface in or very close to the focal plane path of the scanned beam. SOS pulses from the detector assembly are returned via coaxial cable or twisted pair to a central electronic network containing the image data, system timing, and laser modulation circuitry.
A third detection method is known in the art wherein the position of a scanning laser beam is sensed by placing an optical fiber or light pipe in the path of the scanning beam to transmit the incident light to the central electronics system. The conveyed light energy is incident on an indicia as, for example, disclosed in U.S. Pat. No. 4,071,754, or on a photodetector, located on a central circuit board of the electronics system. The detector converts the light energy into an electrical signal which is then processed to provide synchronization signals for the laser. See also the fiber optic detector disclosed in co-pending application Ser. No. 08/217,822, filed on 25 Mar. 1994, entitled, "FIBER OPTIC SCANNING BEAM DETECTOR" and now U.S. Pat. No. 5,444,293, assigned to the same assignee as the present invention. The system disclosed in this application uses a single optical fiber positioned in the scan path at the beginning of a scan line sweep. The fiber transmits a portion of the scan beam flux to a photodetector located on a central electronics circuit board. The photodiode generates an output signal which drives one input of a high speed comparator. The second comparator input is fed an amplified and delayed analog of the photodetector output signal. The comparator senses the difference in the two output wave forms and generates an output transition signal at the precise time the two wave forms cross over or intersect. The comparator output transition signal is used to initiate the scanning system SOS signal of a gas or laser diode ROS.
Fiber optic detectors have several advantages over the split and slit detector arrangements; they are more compact, less expensive, provide superior noise immunity and have simple mechanical mounting. Lower cost is realized because separate scan detector circuit boards and housings are not needed and because the cable and connectors that provide power and signals to and from the remote scan detector board are unnecessary. A dual element photodiode comprising a pair of closely spaced photosensitive elements in a common package suitable for use in a split detection design is also more expensive than an equivalent single element photodiode device. Noise immunity is superior because the laser printer environment is electrically noisy (EMI, RFI) and the remote scan detector and its cables are difficult to shield from this noisy environment. The optical fiber simply acts as a light flux conduit through the noisy environment to the main system electronics board where the light signal is converted to an electronic signal in a controlled environment (shielded) where signal traces are short and noise is easier to control. Further, the mechanical mounting of the remote detector system is often awkward because the scanner footprint is typically narrow near the ROS image plane and the space available for the scan detector electronics board is relatively cramped.
It would be desirable to use a single channel optical fiber detection system which employs less complex circuitry than that required in the prior art. An embodiment is therefore disclosed which is directed to a means and method for generating a start of scan signal using a single fiber optic detection channel which functions much as a differential or split detector, but which does not require linear pulse delay circuitry to create an undistorted delayed analog of a first detected signal. This SOS signal generation is accomplished by using a single optical fiber or light pipe coupled with circuitry that forms a voltage wave form which is intersected or crossed over by the trailing edge of the initially detected signal wave form. More particularly the present invention relates-to a fiber optic scanning beam detector comprising:
fiber optic means positioned in the path of a periodically sweeping beam of light, said fiber optic means transmitting all or a portion of said intercepted light to a photodetector thereby causing said photodetector to generate a voltage wave form V1 having a leading and trailing edge and amplitude A corresponding to the intensity of said intercepted light,
circuit means for forming a second voltage wave form V2, having a leading edge, extended decay, and peak amplitude B proportionately less than that of amplitude A of wave form V1, and
comparator means for comparing said first and second voltage wave forms V1 and V2 and for generating an output transition signal V3, upon detection of the trailing edge of V1 crossing V2.